The present invention relates to a colored forgery prevention structure and a colored forgery prevention medium which exhibit a high forgery prevention effect and a high alteration prevention effect.
A forgery prevention structure is used to prevent forgery of valuable papers, brand-name products, certificates, and personal authentication media, and functions to prove articles to be genuine.
In recent years, a forgery prevention structure using an optical element such as a diffraction grating or a hologram is applied to various articles because a special optical effect thereof can be distinguished at a glance. A lot of optical elements include a micro structure, such as a diffraction grating, a hologram, or a lens array. These micro structures are difficult to analyze. In addition, an optical element including a micro structure is manufactured using an electron-beam lithography apparatus, and can therefore exhibit an excellent forgery prevention effect.
However, a hologram which has a silver metallic luster has been widely marketed for a packaging purpose and an amusement purpose. Therefore, the security thereof is deteriorating. To respond to these circumstances, for example, Patent Literature 1 proposes a hologram which partially comprises a reflection layer, as a hologram which provides a forgery prevention effect. The hologram which includes a reflection layer having a micro pattern is considered difficult to counterfeit.
On the other hand, a proposal has been made for a hologram which has a metallic luster in a vivid color tone other than silver. For example, Patent Literature 1 proposes a hologram structure with which a desired color tone is obtained and which is excellent in design and/or security. Color tones can be more or less chosen by using this method. However, the reflectance of a high-intensity ink layer is lower than that of a vapor-deposited film of aluminum and has a tendency to scatter. Therefore, there is a defect that a colored metallic luster, e.g., a metal luster of a vivid color tone such as gold or copper is not obtained.
Further, a partially colored reflection layer is difficult to provide on a hologram through the method according to Patent Literature 1. This defect is caused by the difficulty in overprinting a reflection layer of a high-luminance ink in an identical pattern over a colored layer with high positional accuracy, after pattern-printing of the colored layer.
Another method has been considered in which a vivid color reflection layer is formed of gold or copper by a vacuum deposition method or a sputtering method, and patterned by etching. However, forming a precious-metal reflection layer by etching requires high costs and exhibits low productivity.